Ultraportable notebook computers are thin, lightweight and hot, in more ways than one.

The rap on them has been that they're usually too thin to have room for floppy and DVD or CD-ROM drives, let alone a good-size battery. They've been called lightweights in performance, storage and battery life compared with desktop-replacement notebook PCs.

And the case bottoms of ultraportables'also known as ultralights and ultramobiles'get hot to the touch, seemingly more so than other notebooks. This can be a little dangerous because their lightness makes them tempting to use on your lap.

The newest ultraportables still suffer from many of the same compromises, but they manage them better. Ultralow-voltage (ULV) or LV CPUs from Intel Corp. and its mobile computing rival, Transmeta Corp. of Santa Clara, Calif., extend battery life and reduce heat.

Flexible lithium-polymer batteries and older lithium-ion technology have been steadily improving, boosting battery life to nearly four hours and allowing for lightweight second batteries that extend run time to a full workday before recharging. By attaching along the bottom rear of the case, the batteries even improve the typing angle.

But the big trend in ultraportables is built-in wireless. Most notebooks of less than 4 pounds'the industry's rough definition of an ultraportable'now come with network circuitry and transmitters for IEEE 802.11b, the wireless LAN standard called WiFi for short.[IMGCAP(2)] A less common version popular in corporations, 802.11a, is an option on some models, notably from Hewlett-Packard Co., IBM Corp. and Toshiba America Information Systems Inc. The faster, emerging 802.11g will be available via software upgrades, according to these vendors.

WiFi lets you browse the Web and check your e-mail at broadband speed, and even view data securely on agency servers near public hot spots or campus base stations.

WiFi helps mitigate ultraportables' storage and processor compromises by letting you rely more on the Web and enterprise servers for applications that would otherwise demand on-board capacity. In that sense, the ultraportable becomes sort of a mobile thin client.

'It makes the ultralights more popular,' said Tim Bajarin, president of market researcher Creative Strategies Inc. of Campbell, Calif. Bajarin added, however, that ultraportables' single-spindle designs'most have space for only one internal drive'make them less popular than roomier, two-drive thin-and-light systems such as Dell Inc.'s Latitude C600.

Product managers agree people still use ultraportables as real PCs, except for the smallest, scrunched-screen-and-keyboard systems that function like the defunct subcategory of so-called PC companions such as Toshiba's discontinued Libretto.

Bajarin said they might make a slight comeback: The new 3/4-inch-thin Actius MM10 from Sharp Systems of America, unlike most ultralights, stands up in an included Universal Serial Bus docking cradle and has Iomega Corp. file-synchronization software.

New CPUs geared to the power and space constraints of mobile PCs are the other major innovation affecting ultraportable design. Transmeta, in fact, was founded in 2000 expressly to beat market-leading Intel in portables.

Its latest offering, the Crusoe TM5800, uses software emulation to run at respectable if unspectacular speeds of up to 1 GHz, but at a fraction of the voltage of Intel's faster Mobile Pentium III Processor M.

Crusoe, however, appears only in the thinnest, under-3-pound ultraportables and Tablet PCs whose battery life is more important than power. These include the MM10, Fujitsu's LifeBook P1000 and several models sold in Asia.

An upgraded processor with improved graphics and support for 400-MHz double-date-rate memory'faster than the current 266-MHz DDR'and code-named Astro, will ship by the end of the month, offering 'dramatically increased performance from what we're getting with the 5800,' Transmeta marketing director Mike DeNeffe said.

Not to be outdone, Intel last year released the confusingly named Pentium M, which runs at up to 1.7 GHz and has a faster system bus and larger memory cache, though ULV versions with longer battery life run at only 900 MHz.

This March, the company added low-power chip sets for graphics and WiFi plus a controller for 266-MHz DDR memory, branding the package Centrino. Most vendors now use the Pentium M but often opt out of the Centrino chip set for better alternatives.

Many models, however, come with the previous-generation Pentium III Processor M, which currently tops out at 1.33 GHz.